At present, optical fiber communication networks are expanding from trunk systems to ordinary homes, and are gaining wide recognition as FTTH (Fiber To The Home) services. Nearly all the optical fibers employed in such communication networks are silica-based fibers, and has a total reflection-type optical waveguide structure provided with a greater refractive index difference with respect to the cladding region by doping, for example, GeO2 or another refractive index increaser to the core region. In particular, the above FTTH services are realized through so-called PON (Passive Optical Network) systems, in which subscribers share a single optical fiber by interposing a multistage optical splitter between the interior of a terminal station which is a final relay station of an existing communication system such as the Internet and the like, and the interval from the terminal station to subscriber homes.
However, in a PON system in which a plurality of subscribers share a single optical fiber by interposing a multistage optical splitter as described above, it is a fact that there are technical problems with respect to future increases in transmission capacity, such as congestion control and securing reception dynamic range. As one means of resolving these technical problems (congestion control, securing of dynamic range, and the like), movement to a SS (Single Star) system is conceivable. When moving to a SS system, the number of fiber cores on the station side is increased compared with a PON system, and so station-side optical cables with extremely small diameters and ultra-high densities are essential. Multi-core optical fibers are suitable as extremely small-diameter, ultra-high density optical fibers which answer such demands.
A multi-core optical fiber is an optical fiber having a plurality of cores, each of which functions as an optically independent optical waveguide. However, because the core regions are in proximity with each other, in a state in which high-power light propagates in each core, crosstalk occurs, arising from the propagation of light which has leaked from each of the cores when small-diameter bending is applied, and, when the fiber length is long, arising from a portion of light propagating outside the core regions even in a state in which no bending is applied. Hence, in for example Non-patent Reference 1 below, a design example of a multi-core optical fiber with a target value for crosstalk between core regions of −30 dB or lower is disclosed in FIG. 3, and in a design example with a relative refractive index difference Δ of 1.2% (FIG. 3(c)), a multi-core optical fiber with 19 core regions is proposed. In this specification, light contributing to crosstalk between core regions, regardless of the origin of the occurrence thereof, is hereafter called leakage light.